Release of [3H]dopamine from guinea pig striatal slices is modulated by sigma1 receptor agonists

Sigma receptors are found in motor and limbic areas in the brains of humans, non-human primates, and rodents. The most extensive pharmacological studies of ligand binding to sigma receptors have utilized brain tissue from guinea pigs, where two subtypes of sigma receptor, designated sigma1 and sigma2, have been identified. Few functional roles for sigma receptors have been described. Their location in guinea pig striatum, a terminal field of dopaminergic projections arising from the substantia nigra, suggested that this tissue would be a logical choice in which to examine physiological properties of sigma receptor activation. We found that sigma1 receptor agonists inhibited N-methyl-D-aspartate-stimulated [3H]dopamine release from guinea pig striatal slices in a concentration-dependent manner. The inhibition by sigma1 receptor agonists was reversed by a selective sigma1 receptor antagonist, as well as by a non-subtype-selective sigma receptor antagonist. The ability of agonists working through sigma1 receptors, but not through sigma2 receptors, to inhibit the stimulated release of catecholamines appears to be a unique characteristic of guinea pig striatum. We have previously reported that in rat striatum and hippocampus, as well as in guinea pig nucleus accumbens, prefrontal cortex, and hippocampus, activation of either sigma receptor subtype inhibits such release. Stimulated release of [3H]dopamine from guinea pig striatum was also inhibited by the phencyclidine receptor agonist dizocilpine, but this inhibition was not reversed by the sigma receptor antagonists. Therefore, the inhibition produced by sigma receptor agonists was not mediated via the phencyclidine binding site within the N-methyl-D-aspartate-operated cation channel. Our findings support the hypothesis that sigma receptor activation provides a mechanism of modulating dopamine release from striatum, and that striatal tissue from guinea pigs appears to be an appropriate model for characterizing sigma1 receptor-mediated effects.